DNA purified from cells (either avian or mouse) transformed by B77-ASV will be digested to completion with restriction endonuclease of known specificity. Products of endonuclease digestion will be separated by agarose gel electrophoresis systems so as to allow maximum separation of fragments with varying length. The separated fragments will be denatured and transferred to a cellulose nitrate strip using the technique of Southern. Fragments containing virus-specific sequences will be localized by hybridizing the cellulose nitrate strips with (32P)-DNA complementary to B77-ASV virus RNA. Unhybridized (32P)-cDNA will be recovered by washing E (and possibly by S1 digestion) and the hybrid containing fragments visualized by autoradiography and quantitated by radioactive counting.